The Collaborative International Dictionary of English v.0.48:

Semitone \Sem"i*tone\, n. [Pref. semi- + tone. CF. Hemitone.]
   (Mus.)
   Half a tone; -- the name commonly applied to the smaller
   intervals of the diatonic scale.
   [1913 Webster]

   Note: There is an impropriety in the use of this word, and
         half step is now preferred. See Tone. --J. S. Dwight.
         [1913 Webster]

WordNet (r) 3.0 (2006):

semitone
    n 1: the musical interval between adjacent keys on a keyboard
         instrument [syn: semitone, half step]